Master base for fabrication and method for manufacturing the same

ABSTRACT

A master base for fabrication includes a substrate, a first photoresist layer disposed on the substrate, and a second photoresist layer disposed on the first photoresist layer, wherein the first photoresist layer attenuates or absorbs rays reflected at the interface between the first photoresist layer and the substrate to prevent the reflected rays from interfering with applied rays in a exposing step. A method for manufacturing a master base for fabrication includes the steps of forming a first photoresist layer on a substrate, baking the first photoresist layer at the setting temperature of the first photoresist layer, and forming a second photoresist layer on the first photoresist layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to master bases for fabrication, forexample, a master base for manufacturing masters for the fabrication ofmicrolens arrays, and a method for manufacturing the master base forfabrication.

2. Description of the Related Art

Currently, the following processing procedure is generally employed formanufacturing microlenses: a photoresist material is applied onto asubstrate to form a photoresist layer thereon, the photoresist layer ispatterned to form an array, and the obtained array is provided as a moldfor microlenses.

In this processing procedure, as shown in FIG. 3, a single photoresistlayer 20 is formed on a substrate 10, and the photoresist layer 20 ispatterned to form a desired pattern by a photolithography method.However, there is a problem in that the obtained pattern does not have asmooth surface functioning as, for example, a lens surface. That is,steps 30 are situated on the surface of the pattern, as shown in FIG. 3Band FIG. 3C, which is a partly enlarged view of FIG. 3B.

It has been found that the above phenomenon is caused as follows: in asingle photoresist layer on a substrate, rays applied to the photoresistlayer interfere with rays reflected at an interface between thesubstrate surface and the photoresist layer surface to form standingwaves and the photoresist layer is also exposed to the standing waves.

That is, when the single photoresist layer is exposed, the applied rays,which are supplied from a light source, interfere with the reflectedrays to form other rays having different intensities. The formed rayshaving different intensities cause steps, which form the same pattern asthe interference pattern, on the surface of the photoresist layer. As aresult, the processed photoresist layer having an optical lens shapehave steps on the surface, that is, the surface is not smooth.Therefore, when the processed photoresist layer is used as a master formanufacturing, for example, microlenses, obtained microlens haveinferior converging performance due to the occurrence of scattering.

In the present invention, in order to solve the above problem, two ormore photoresist layers are formed on a substrate to suppress oreliminate the adverse effects caused by rays reflected at the interfacebetween the bottom of the photoresist layers and the substrate, asdescribed below. However, this method employing two or more photoresistlayers causes another problem. That is, in a step of forming a pluralityof the photoresist layers, a photoresist layer is formed and is thenpre-baked to form another photoresist layer thereon, and this procedureis repeated. In this procedure, when the formed lower photoresist layeris pre-baked at a temperature of less than 120° C., which is included inthe pre-baking conditions of photoresist layers used in an ordinaryphotolithography method, the formed lower photoresist layer is notsufficiently cured. Therefore, when the upper photoresist layer isprovided on the lower photoresist layer in this state to form a layeredphotoresist portion, the treatment of providing the upper photoresistlayer causes the thickness of the lower photoresist layer to change sothat the thickness of the entire layered photoresist portion changes. Asa result, there is a problem in that the difference in thickness isincreased depending on areas of the layered photoresist portion.

The difference in thickness causes a visible interference pattern overthe upper surface of the layered photoresist portion.

FIG. 4 is an illustration showing a process in which the difference inthickness arises in the layered photoresist portion. As shown in FIG.4A, a first photoresist layer 21 is formed on a substrate 10. As shownin FIG. 4B, the first photoresist layer 21 is then pre-baked at lessthan 120° C. When an second photoresist layer 22 is provided on thefirst photoresist layer 21 in this state, the flatness of the firstphotoresist layer 21 is damaged by the formation of the secondphotoresist layer 22, thereby causing waves over the top surface, asshown in FIG. 4C. This state causes the visible interference patterndescribed above.

If a master base prepared by the above method is used for manufacturinglenses, optical lenses having a desired performance cannot be obtained.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amaster base for fabrication in which reflected rays are attenuated orabsorbed to prevent applied rays from interfering with the reflectedrays so that rays having different intensities are not formed.

It is another object of the present invention to provide a method formanufacturing a master base for fabrication having high processingpreciseness, wherein the method includes the step of forming a pluralityof photoresist layers and the influence of a upper sublayer on a lowersublayer is suppressed in the step to reduce the difference in thicknessof the amount of the photoresist layers.

In order to achieve the above objects, the present invention provides amaster base for fabrication including a substrate, a first photoresistlayer disposed on the substrate, and a second photoresist layer disposedon the first photoresist layer, wherein the first photoresist layerattenuates or absorbs rays reflected at the interface between the firstphotoresist layer and the substrate to prevent the reflected rays frominterfering with applied rays in an exposing step.

In the master base for fabrication of the present invention, the firstphotoresist layer may include a plurality of sublayers.

Furthermore, the present invention provides a method for manufacturing amaster base for fabrication including the steps of forming a firstphotoresist layer on a substrate, baking the first photoresist layer atthe setting temperature of the first photoresist layer, and forming asecond photoresist layer on the first photoresist layer.

In the method of manufacturing a master base for fabrication of thepresent invention, the first photoresist layer may include a pluralityof sublayers, and the plurality of sublayers may be prepared byrepeating the steps of forming a sublayer comprising a photoresistmaterial to bake the sublayer at the setting temperature of thephotoresist material.

Since a layered structure having two or more photoresist layers isemployed, rays reflected at the interface between the substrate and thefirst photoresist layer are reduced, thereby obtaining a smooth surface,which is spherical or a spherical.

Since a lower resist layer is pre-baked at a setting temperature thereofor more, for example, 120° C. or more, to set the lower photoresistlayer sufficiently, the difference in thickness of the layered structureis reduced when an upper photoresist layer is formed on the lowerphotoresist layer, thereby eliminating a visible interference pattern.Thus, a master base for fabrication having superior processingpreciseness can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a process for manufacturing a masterbase for fabrication according to the present invention;

FIG. 2 is an illustration showing a method of forming photoresist layersfor manufacturing a master base for fabrication according to the presentinvention;

FIG. 3 is an illustration showing a process for manufacturing aconventional master base for fabrication; and

FIG. 4 is an illustration showing a step of forming photoresist layersfor manufacturing a conventional master base for fabrication.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described withreference to accompanying drawings.

FIG. 1 is an illustration showing a process for manufacturing a masterfor fabrication, for example, a microlens master.

In this embodiment, in order to eliminate surface steps caused byinterference due to applied rays and reflected rays in an exposure step,a first photoresist layer 21 is disposed on a substrate 10 and a secondphotoresist layer 22 is disposed on the first photoresist layer 21, asshown in FIG. 1. The first photoresist layer 21 attenuates or absorbsrays reflected at the interface between the substrate 10 and the firstphotoresist layer 21 to prevent the reflected rays from interfering withapplied rays for exposure. Thus, a lens array 31 having a smoothsurface, that is, a surface with no steps, can be obtained. Two or morephotoresist layers may be disposed on the substrate 10.

In order to form a layer having a large thickness, a material for thelayer having a large viscosity must be used. However, when such amaterial is used, it is difficult to form a layer having an eventhickness because the material is not uniformly spread. Therefore, thefirst photoresist layer 21 includes a plurality of sublayers having athickness of 2 to 10 μm so as to have a thickness sufficient toattenuate or absorb the reflected rays.

FIG. 2 is an illustration showing a method of forming photoresist layerson a substrate according to an embodiment of the present invention.

As shown in FIG. 2A, a photoresist material is applied onto thesubstrate 10 to form the first photoresist layer 21. The firstphotoresist layer 21 is then pre-baked at a temperature at which thephotoresist material is sufficiently set, for example, 120° C. or more.In this embodiment, the second photoresist layer 22 is formed on thefirst photoresist layer 21 after the first photoresist layer 21 iscompletely set.

The completely set first photoresist layer 21 can maintain the flatnessof the surface when the second photoresist layer 22 is formed thereon.Therefore, the second photoresist layer 22 has a flat surface, as shownin FIG. 2C.

A master base for fabrication of the present invention is thencompleted. The obtained master base for fabrication is patterned byexposure and development into a master having the lens array 31 with asmooth surface.

When such a master is used for manufacturing microlenses, microlenseshaving desired optical characteristics such as a light-convergingfunction can be obtained.

1-2. (canceled)
 3. A method for manufacturing a master base forfabrication comprising the steps of: forming a first photoresist layeron a substrate; baking the first photoresist layer at the settingtemperature of the first photoresist layer, and forming a secondphotoresist layer on the first photoresist layer.
 4. The method formanufacturing a master base for fabrication according to claim 3,wherein the first photoresist layer comprises a plurality of sublayers,and the plurality of sublayers are prepared by repeating the steps offorming a sublayer comprising a photoresist material to bake thesublayer at the setting temperature of the photoresist material.